As is well known, bar codes have come into wide use for marking an incredible variety of objects for automatic reading. A substantial technology has developed for reading such bar codes to have a high first-read rate and a low substitution-error rate. Both the optical and electrical characteristics of the readers and the decoding systems have been the subject of extensive development efforts.
One type of optical system for a bar code reader employs a modified Scheimpflug arrangement. A thin fan of light illuminates a line across the bar code. When the bar code is within the working range (i.e. depth of field) of the scanner, that line is focused on an array of photodetectors. The photodetectors are sampled sequentially to produce a train of signals representative of the illumination of each photodetector. This analog signal goes to a digitizer and decoder for identifying bars and spaces and generating an output digital signal representative of the data recorded in the bar code.
One such Scheimpflug optical arrangement employs what is essentially a one-dimensional array of elongated photodetectors with the illuminated line on the bar code imaged along the length of the array. A single train of signals is obtained from such a photodetector array.
In another Scheimpflug optical system the illuminated line is focused on a two-dimensional array of photodetectors. This array is electronically scanned in a raster pattern for generating what amounts to a video signal with a plurality of "horizontal" lines across the "vertical" extent of the field. The lines are scanned sequentially, producing a train of many signals, each representing a line in the array. Those signals may then be decoded to read the bar code.
It is a characteristic of the Scheimpflug optical arrangement that there is one line in the array on which the bar code is best focused. If the bar code happens to be in about the middle of the working range of the scanner, the line having the best focus will be in about the middle of the raster pattern. If the bar code happens to be near one extreme of the working range, the image of the bar code that is in best focus will be near one extreme of the array and the signal corresponding to the line of best focus will be near the beginning or end of the train of signals representing the raster pattern. Other areas on the array may also be illuminated by the light reflected from the bar code and the focus of the bar code in those areas is poorer than the line of sharpest focus.
Sometimes there is not a single "best" focused line in the raster pattern, but a few lines may be more or less equally in focus. That is not significant in practice of this invention and any of such lines may be regarded as the "best".
It is desirable to reduce the data processing requirements of a decoder employed in such a system. The data produced during the raster scan of the entire array of photodetectors may amount to 875 kilobits in a conventional TV format or up to four megabits in a more advanced system, a formidable processing task for finding the line that is in best focus, and a large amount of memory to be devoted to storing the data until the line with sharpest focus is found. It is desirable to find the best focused line with analog processing rather than the digital processing presently employed. This permits less costly decoders or reduces delay in processing the signals and produces an output nearer to real time.
Thus, it is desirable to select a line within the raster pattern to be processed by the decoder with other lines in the array being ignored. The quantity of data to be processed can thereby be reduced, with consequent savings and without detriment to the first-read rate or substitution-error rate.
Because of the broad working range of the Scheimpflug optics it may happen that more than one bar code is imaged on the photodetector if more than one happens to be in the field of view of the scanner. This may lead to ambiguity and error. It is therefore desirable to provide means for distinguishing a primary bar code from other signals that may appear in the field of view. Again it is desirable that this be done as early in the signal processing as possible for minimizing data processing requirements and providing output near real time.